The present invention relates to a method and a device for supplying voltage to a three-phase voltage system having a load-carrying neutral conductor. More particularly, the present invention relates to regulators for controlling such a system.
German Application No. DE-A-32 36 071, shows the options of supplying synchronous voltage to a three-phase system or supplying a synchronous voltage to a secondary distribution network through an invertor. The invertor can be, in particular, a pulse-controlled invertor having an input for controlling the amplitude and frequency of the output voltage. A controlled variable for the frequency can be supplied by a superposed active-current regulator, and a controlled variable for the amplitude can be supplied by a superposed reactive-current regulator. The control of frequency and amplitude by the invertor stems from the generation of a symmetrical and zero-sequence-free system of invertor output voltages. This frequency and amplitude control system can only compensate for asymmetrical load surges with difficulty or not at all, especially when the load is connected asymmetrically between the phase conductors and a load-carrying neutral point.
As is shown in U.S. Pat. No. 4,367,522, in supplying a three-phase voltage system having a connected neutral point, each individual phase conductor of the three-phase voltage system can be fed by its own 4-pulse bridge circuit (B4 invertor) having a separate control device. The three setpoint voltage values of these control devices are specified as a three-phase, symmetrical system of reference vectors. Each invertor is switched to the primary side of a single-phase transformer without forming a connected neutral point. The secondary sides are joined at one end to a load-carrying neutral point which can be connected to the neutral conductor. The secondary sides are joined at their other ends to the phase conductor of the three-phase voltage system. This transformer device is augmented by a filter comprised of inductors and capacitors, preferably short-circuit limiting reactors and shunt capacitors. This method quickly compensates for harmonic voltages, which are generated by harmonic currents having a low harmonic number due to non-linear loads at the internal resistance of the employed invertor configuration. The total harmonic content can be kept at less than 5%. Load surges can also be quickly compensated using this method. Thus, with this method, any system can be supplied having three line voltages and respective conductor currents. However, the power circuitry is relatively high in cost due to the three separate bridge invertors and the required regulating component for this circuitry.
As shown in U.S. Pat. No. 4 719 557, a 6-pulse bridge circuit is used to feed a three-phase voltage system. One bridge-arm pair, each pair comprising two in-line invertor valves, is connected at its junction point to one input of a transformer device. The other connecting terminals of each bridge-arm pair are coupled to a defined input d.c. voltage. The primary-side neutral point of the transformer device is not coupled to other parts of the installation or is not provided at all. This method allows for considerable savings on the cost of installation. With these earlier methods, three separate voltage-control devices form three individual, pulse-width modulated actuating signals from the three measured values of the line voltages and the three zero-sequence-free, symmetrical, setpoint voltage values for the bridge-arm pairs. In the closed-loop control system of the voltage-control devices, a conversion is undertaken from secondary-side line voltages to primary-side voltages corresponding to the structure of the transformer device. Since, however, the invertor only allows a zero-sequence-free system of currents and voltages to be supplied, the three separate control devices are redundant in the system. Therefore, these three controllers affect each other and interfere with the control response of the system. Instabilities can occur if a proportional-action controller is not used.
There is a need for a method that uses only two voltage regulators, whose setpoint values characterize a symmetrical, zero-point-free system, and to compare only two actual voltage values. The pulse-width modulated output signals of the two regulators control two of the three bridge-arm pairs. The third bridge-arm pair is controlled by a signal that is formed through the appropriate summation of the two other control signals and through pulse-width modulation.
Using this method, a symmetrical voltage system can be maintained in a passive network, in spite of quickly changing and asymmetrical loads. Thus, the currents flowing into the network freely adapt to the changing loads.
If, however, the three-phase voltage system is already supplied by other voltage sources (e.g., a supply system and/or other invertors), it is often desired to control or regulate the distribution of the currents flowing into the network.
Therefore, there is also a need for a symmetrical voltage system in a three-phase voltage system having a load-carrying neutral conductor, even when the three-phase voltage system is supplied in parallel by another voltage source. There is also a need for a voltage system that is maintained with minimal expenditure.